A boiler tube and turbine for a power plant, a nuclear power plant, and a chemical plant facility are used for a long time in high-temperature and high-pressure environments. Accordingly, superb strength, corrosion resistance, and oxidation resistance at elevated temperatures and high toughness at room temperature are required for these equipments.
Recently, from the viewpoint of prevention of global warming, improvement of thermal efficiency is required to reduce emission of CO2 in thermal power plants, and operation conditions in terms of temperature and pressure become significantly high in the thermal power plant boiler. For example, new plants are being built one after another with operation conditions comprising a temperature of exceeding 600° C. and a pressure of 300 atm. For materials to be used for many hours at high temperatures, it is necessary to ensure creep characteristics. However, the above operation conditions are extremely hostile for heat-resistant steels.
On the other hand, upon a request of relaxation of regulations from home and abroad, marketing is liberated in electricity business, so that firms other than electric power companies or trading houses can enter the electricity business. As a result of severe price competition, economic efficiency is highly regarded than ever before in the power plant.
Additionally, research and development for maintenance of the facilities at low costs without the risk of safety becomes important in not only the new power plants but also aging facilities. Under these circumstances, there arises a growing demand for a heat-resistant steel in which, despite being achieved at low costs, high-temperature strength is enhanced compared with the conventional steels, and development for a high strength material which can respond to the demand is in progress.
Conventionally, Cr—Mo low alloy steels such as JIS G3462 STBA22 (1Cr-0.5Mo steel), JIS G3462 STBA23 (1.25Cr-0.5Mo steel), and JIS G3462 STBA24 (2.25Cr-1Mo steel) are used in a relatively-low temperature range up to about 550° C. Recently, in order to enhance high-temperature creep strength, a steel in which part of Mo is replaced by W (for example, steel disclosed in Japanese Patent Application Publication No. 8-134584) and a steel in which hardenability is significantly enhanced by addition of Co (for example, steel disclosed in Japanese Patent Application Publication No. 9-268343) are developed.
In such newly developed steels, softening resistance is improved at high temperatures by W or Co, and creep strength is particularly improved at not less than 500° C. compared with conventional general-purpose steels. However, it is obvious that, because of pursuing high strength, deterioration of toughness and a decrease in long-term creep ductility (elongation and reduction of area) become prominent.
In order to prevent the deterioration of toughness and to improve the creep ductility, there is proposed a steel in which V, Nb, and Ti are added to the Cr—Mo steel (for example, a steel disclosed in Japanese Patent Application Publication No. 2004-1077191). However, in the steel disclosed in Japanese Patent Application Publication No. 2004-107719, although the toughness is improved, there is further room for improvement in compatibility between the high-temperature creep strength and the creep ductility.